The magnetizable layers of most magnetic recording media consist of coatings of fine magnetizable particles in organic binder. The magnetizable particles typically have a maximum dimension of less than 1 .mu.m. A magnetizable coating should be of uniform thickness, typically less than 1.0 mil (25 .mu.m), and should be free from defects such as pinholes, streaks, and particle agglomerates. The coatings can be applied by a direct gravure coater as illustrated in U.S. Pat. No. 3,761,311. This patent shows that before significant evaporation of the coating on a backing member, the knurl pattern on the coating (from the gravure cylinder) is smoothed out by a flexible blade, known as a drag smoother. This patent does not explain the smoothing operation of the flexible blade or give any information as to its construction.
With conventional, straight-edged smoothers, removing gravure patterns on thin coated substrates is difficult without distorting the substrate at the smoothing station, where the substrate travels in a free span between idler rollers. Any distortions cause non-uniformities in coating thickness, wound-in stresses, and related defects that degrade the performance of advanced magnetic tape products. This is true for both the magnetic coatings and the backside coatings, some of which use carbon-black particles in a binder system to improve conductivity, runnability, air bleed, winding properties, and other properties. There is a need to manage substrate instabilities at the point of smoothing and several improved versions of drag smoothers are known.
In one version, the drag smoother is a trailing flexible blade which is pre-tensioned before being clamped in the smoothing assembly so that the originally straight edge of the film becomes puckered or bowed in the cross-web direction, as discussed in U.S. Pat. No. 08/293,481, filed on Aug. 22, 1994 by the assignee of this invention. The back-up smoothing bar is a flat, rigid, rectangular plate, with an outer edge which is straight and undeformable perpendicular to the direction of web travel. To achieve bowing and attenuate web wrinkles or flutter, a second, curved flexible film 28 is used. The resulting shape imparts tension variations on the substrate that flatten out the tension troughs which thin polyester films and other tensioned substrates tend to form between two idler rolls. This improves smoothing quality. Although very effective for conventional magnetic tape products on polyester substrates thicker than 10-12 microns, the puckered film method does not produce the desired results on thin products with substrate thicknesses of 8 microns or less.
In another drag smoother discussed in U.S. Pat. No. 5,447,747, a flexible polyester film with a curved edge is placed between a rigid back-up plate and a smoothing film, both having straight edges, or is clamped on top of the rigid back-up plate. In either configuration, the curved edge bows the smoothing film toward the web in the center and reduces substrate buckling, even on substrates between 4 and 8 microns thick. A drawback of this system is its inability to readily accommodate different amounts of bowing. The optimal amount of bowing depends on coating conditions such as coating thickness, speed, and viscosity, on substrate properties such as thickness and modulus, and on variations in substrate quality from roll to roll.
Japanese Reference 50-22835 discusses smoothing wet coatings of magnetizable particles that have been applied by gravure coating. It discusses smoothing sheets of plastic film, with the working edges either curved or straight except for diagonal corners. Both types improve smoothing compared to a rectangular sheet. This reference concludes that thinner smoothing sheets are more effective because a thinner smoothing sheet makes better contact with the coated backing member. These configurations seek to cause tension variations and may be equally effective in flattening the substrate at the point of smoothing. A major drawback, however, is the need to cut the polyester film into curved or slanted shapes, which is prone to cause burrs or other edge defects which can produce streaks and other coating flaws.
U.S. Pat. No. 4,547,393 discloses a flexible sheet for smoothing a coating of highly viscous solution. A flexible sheet smoothes while the coating is subjected to a magnetic field. The flexible sheet is from 4 to 120 microns thick. The flexible sheet can be mounted on a pedestal whose position prevents longitudinal lines or streaks from forming on the sheet. When the flexible smoothing sheet is mounted on a roll, the contact area of the sheet at the smoothening area can be adjusted by turning the roll and preventing longitudinal lines or streaks occurring on a sheet by adjusting the contact area.
U.S. Pat. No. 4,870,920 uses a rigid smoothing bar to smooth wet coatings of magnetizable particles. A stiff bar-shaped or deformable plate-shaped member having a smooth surface is used. It also describes using a combination of the flexible sheet and bar.
These last two patents fail to explicitly address stabilizing thin gauge substrates at the point of smoothing. U.S. Pat. Nos. 4,995,339 and 5,042,442 use an air nozzle system or crowned idler rollers to stabilize thin substrates traveling over fluid-bearing coating dies. While these concepts may be applicable to smoothing with a flexible, trailing blade, they are very complex and not very flexible.
Japanese Reference 60-57387 uses a solid body smoother 3 to smooth wet coatings of magnetizable particles.